1. Field of the Invention
The present invention is in the field of early warning devices for fire detection and more particularly relates to a compact apparatus comprising a conventional smoke detector, preferably a photoelectric type, working in conjunction with a tuned odor detector to suppress false alarms and to detect more rapidly the onset of a fire.
2. The Prior Art
The fire detectors that are available commercially today fall into three basic classifications, namely flame sensing, temperature sensing, and smoke particle sensing. This classification is designed to respond to the three principal types of energy and matter characteristics of a fire environment: flame, heat and smoke. A fourth class of fire detectors was advanced almost a decade ago (U.S. Pat. No. 5,053,754), although still not yet commercially exploited today, which measures the concentration and the rate of change in the concentration of carbon dioxide gas, a principal byproduct of fire combustion, at the onset of a fire as a means for early and rapid detection of it.
The flame sensing detector is designed to respond to the optical radiant energy generated by the diffusion flame combustion process--the illumination intensity and the frequency of flame modulation. Two types of flame detectors are commonly in use: the ultraviolet detectors which operate beyond the visible wavelengths below 4,000 A, and the infrared detectors which operate in the wavelengths above 7,000 A. To prevent false signals from the many sources of ultraviolet, visible and infrared optical radiation present in most hazardous areas, the detectors are programmed to respond only to radiation with frequency modulation within the flicker frequency range for flame (5-30 Hz).
Flame detectors generally work well and seldom generate false alarms. However, they are relatively complex and expensive fire detectors which are not amenable to low-cost and mass-oriented usage. Instead they are mostly utilized in specialized high-value and unique protection areas such as aircraft flight simulators, aircraft hangers, nuclear reactor control rooms, etc.
Thermal detectors are designed to operate from thermal energy output--the heat--of a fire. This heat is dissipated throughout the area by laminar and turbulent convection flow. The latter is induced and regulated by the fire plume thermal column effect of rising heated air and gases above the fire surface. There are two basic types of thermal detectors: the fixed temperature type and the rate-of-rise detector type. The fixed temperature type further divides into the spot type and the line type. The spot detector involves a relatively small fixed unit with a heat-responsive element contained within the unit or spot location of the detector. With the line detector the thermal reactive element is located along a line consisting of thermal-sensitive wiring or tubing. Line detectors can cover a greater portion of the hazardous area than can spot detectors.
Fixed temperature thermal fire detectors rate high on reliability, stability and maintainability but low on sensitivity. In modern buildings with high air flow ventilation and air conditioning systems, placing the fixed temperature detector is a difficult engineering problem. Consequently, this type of thermal fire detector is not widely used outside of very specialized applications.
A rate-of-rise type thermal fire detector is usually installed where a relatively fast-burning fire is expected. The detector operates when the fire plume raises the air temperature within a chamber at a rate above a certain threshold of operation--usually 15.degree. F. per minute. However, if a fire develops very slowly and the rate of temperature rise never exceeds the detector's threshold for operation, the detector may not sense the fire.
The newest type of thermal fire detector is called a rate-compensated detector which is sensitive to the rate of temperature rise as well as to a fixed temperature level which is designed into the detector's temperature rating. Even with this dual approach, the most critical problem for effective operation of thermal fire detectors is the proper placement of detectors relative to the hazardous area and the occupancy environment consequently, this type of fire detector is seldom found in everyday households.
By far the most popular fire detector in use in everyday life today is the smoke detector. Smoke detectors respond to the visible and invisible products of combustion. Visible products of combustion consist primarily of unconsumed carbon and carbon-rich particles; invisible products of combustion consist of solid particles smaller than approximately five (5) microns, various gases, and ions. All smoke detectors can be classified into two basic types: Photoelectric type which responds to visible products of combustion and ionization type which responds to both visible and invisible products of combustion.
The photoelectric type is further divided into 1) projected beam and 2) reflected beam. The projected beam type of smoke detectors generally consist of a series of sampling piping from the holds or other protected space on the ship to the photoelectric detector. The air sample is drawn into the piping system by an electric exhaust pump. The photoelectric detector is usually enclosed in a metal tube with the light source mounted at one end and the photoelectric cell at the other end. This type of detector is rather effective due to the length of the light beam. When the visible smoke is drawn into the tube, the light intensity of the beam received in the photoelectric cell is reduced because it is obscured by the smoke particles. The reduced level of light intensity causes an unbalanced condition in the electrical circuit to the photocell which activates the alarm. The projected beam or smoke obscuration detector is one of the most established types of smoke detectors. In addition to use on ships, these detectors are commonly used to protect high-value compartments or other storage areas, and to provide smoke detection for plenum areas and air ducts.
The reflected light beam smoke detector has the advantage of a very short light beam length, making it adaptable to incorporation in the spot type smoke detector. The projected beam smoke detector discussed earlier becomes more sensitive as the length of the light beam increases, and often a light beam of 5 or 10 feet long is required. However, the reflected light beam type of photoelectric smoke detector is designed to operate with a light beam only 2 or 3 inches in length. A reflected beam visible light smoke detector consists of a light source, a photoelectric cell mounted at right angles to the light source, and a light catcher mounted opposite to the light source.
Ionization type smoke detectors detect both the visible and invisible particle matter generated by the diffusion flame combustion. As indicated previously, visible particulate matter ranges from 4 to 5 microns in size, although smaller particles can be seen as a haze when present in a high mass density. The ionization detector operates most effectively on particles from 1.0 to 0.01 microns in size. There are two basic types of ionization detectors. The first type has a bipolar ionized sampling chamber which is the area formed between two electrodes. A radioactive alpha particle source is also located in this area. The oxygen and nitrogen molecules of air in the chamber are ionized by alpha particles from the radioactive source. The ionized pairs move towards the electrodes of the opposite signs when electrical voltage is applied, and a minute electrical current flow is established across the sample chamber. When combustion particles enter the chamber they attach themselves to the ions. Since the combustion particles have a greater mass, the mobility of the ions now decreases, leading to a reduction of electrical current flow across the sampling chamber. This reduction in electrical current flow initiates the detector alarm.
The second type of ionization smoke detector has a unipolar ionized sample chamber instead of a bipolar one. The only difference between the two types is the location of the area inside the sample chamber that is exposed to the alpha source. In the case of the bipolar type the entire chamber is exposed leading to both positive and negative ions (hence the name bipolar). In the case of the unipolar type only the immediate area adjacent to the positive electrode (anode) is exposed to the alpha source. This results in only one predominant type of ions (negative ions) in the electrical current flow between the electrodes (hence the name unipolar).
Although unipolar and bipolar sampling chambers use different principles of detector design they both operate by the combustion products creating a reduced current flow and thus activating the detector alarm. In general the unipolar design is superior in giving the ionization smoke detectors a greater level of sensitivity and stability, with fewer fluctuations of current flow to cause false signals from variations in temperature, pressure and humidity. Most ionization smoke detectors available commercially today are of the unipolar type.
For the past three decades the ionization smoke detectors have dominated the fire detector market. One of the reasons is that the other two classes of fire detectors, namely the flame sensing detectors and the thermal detectors are appreciably more complex and costlier than the ionization smoke detectors. They are therefore mainly used only in specialized high-value and unique protection areas. In recent years, because of their relatively high cost, even the photoelectric smoke detectors have fallen behind significantly in sales to the ionization type. The ionization types are generally less expensive, easier to use and can operate for a full year with just one 9-volt battery. Today over 90% of households that are equipped with fire detectors use the ionization type smoke detectors. Despite their low cost, relatively maintenance-free operation and wide acceptance by the buying public, the smoke detectors are not without problems and certainly far from being ideal. There are a number of significant drawbacks for the ionization type smoke detectors to operate successfully as early warning fire detectors. Most people do not complain about them simply because there are no better alternatives in their price range.
One of the biggest problems with ionization smoke detectors is their frequent false alarms. By the nature of its operational principle, any micron-size particulate matter other than the smoke from an actual fire can set off the alarm. Kitchen grease particles generated by a hot stove is one classic example. Over-zealous dusting of objects and/or furniture near the detector is another. Steam from a bath room after somebody had taken a bath and opening the bathroom door could also set off the smoke detector mounted in the hallway. Even the accumulation of dust around the smoke detector itself after a long and unattended period of time had elapsed could lead to a false alarm. Frequent false-alarms are not just a harmless nuisance; some people actually disarm their smoke detectors by temporarily removing the battery in order to escape from such annoying episodes. This latter situation could be outright dangerous especially when these people forget to rearm their smoke detectors.
Another significant drawback for the current ionization smoke detector is its relatively slow speed to alert people of a fire. There are several factors that contribute to this particular drawback. The first fact is the detector trigger threshold for smoke which directly affects its response time to the onset of a fire. No doubt a lower trigger threshold would mean a faster fire detector. However, it also means more frequent annoying false alarms for the user. The second factor is the particular placement of the detector with respect to the spot where fire breaks out. Unlike ordinary gases, smoke is actually a complex sooty molecular cluster that consists mostly of carbon. It is much heavier than air and thus diffuses much slower than the gases encountered everyday. Therefore, if the detector happens to be at some distance from the location of the fire, it will be a while before enough smoke gets into the sampling chamber of the smoke detector to trigger the alarm. A third factor is the nature or type of fire itself. Although smoke usually accompanies fire, the amount produced can vary significantly depending upon the composition of the material that catches the fire. For example oxygenated fuels such as ethyl alcohol and acetone give less smoke than the hydrocarbons from which they are derived. Thus, under free-burning conditions oxygenated fuels such as wood and polymethyl methacrylate give substantially less smoke than hydrocarbon polymers such as polyethylene and polystyrene. As a matter of fact a small number of pure fuels namely carbon monoxide, formaldehyde, methaldehyde, formic acid and methyl alcohol burn with non-luminous flames and do not produce smoke at all.
Yet another drawback of present-day ionization smoke detectors has to do with contaminating our environment. Ionization type smoke detectors use a radioactive matter (Co.sup.60) as the source for alpha particles. Although one can argue that the amount of radioactive material currently found in each ionization smoke detector is very small (probably only tens of milligrams) the number of units in operation however easily runs into tens of millions every year. Thus, the continued usage of this type of smoke detector does pose a serious long-term liability of building up a large amount of unwanted nuclear wastes. Since the half life of Co.sup.60 is well over 1,000 years, the potential danger should not be ignored.
Finally, there are a number of lesser issues one has to deal with when using these low-cost ionization smoke detectors. These include the trouble and cost of having to replace its battery once every year or run the risk of owning a unit that does not work because of lack of power. Also, the presently available ionization smoke detectors are rarely equipped with a visual alarm which is desirable for hearing-impaired persons.
Recognizing the fact that effluent gases (most notably carbon dioxide, carbon monoxide and water vapor) invariably accompany a fire environment in addition to flame, heat and smoke, the present inventor in U.S. Pat. Nos. 5,053,754 (1991), 5,079,422 (1992) and 5,103,096 (1992) advanced the idea of measuring both the concentration and the rate of concentration changes for carbon dioxide gas in an enclosed space as a way to detect the onset of a fire. He reasoned that fire initiation is necessarily an oxidation process and carbon dioxide is invariably the principal byproduct of any oxidation along with water vapor. In addition to being generated abundantly right from the start of a fire, carbon dioxide is a very stable gas. Its concentration can easily be measured accurately using Non-Dispersive Infrared (NDIR) techniques that are very advanced at the present time. The average ambient CO.sub.2 concentration level indoors of .about.400-500 ppm (.about.0.04-0.05%) does not hinder the detection of additional fire-induced quantities as long as the carbon dioxide sensor designed as a fire detector has adequately fine sensitivity.
The advent of using carbon dioxide gas to help detect early onset of fires in early 1990's prompted a significant amount of fire detection research using this additional parameter in ensuing years leading up to today. So far the results can best be considered as mixed. There is very little doubt that CO.sub.2 is a very useful parameter for early fire detection. However, the maximum CO.sub.2 concentration detected during experiments with flaming fires are significantly greater than the maximum CO.sub.2 detected during experiments with non-flaming fires (pyrolyzing fires, heated liquids and environmental odors). Based upon these experimental results, it is now generally believed that the CO.sub.2 parameter is best deployed alongside with a conventional smoke detector (either ionization or photoelectric). This way almost 100% of false alarms, which have been a principal drawback for smoke detectors, could be avoided. Furthermore, the speed of response for such a dual fire detector system could be much faster than the smoke detector alone.
Despite these seemingly encouraging test results, such a dual CO.sub.2 /smoke fire detector has yet to make it to the marketplace. The reason for this is two-fold. First, the inclusion of the CO.sub.2 parameter can only be achieved with a rather costly and sophisticated carbon dioxide sensor with relatively high sensitivity, long life and long-term stability. Unfortunately the added cost and complexity are not justified by the albeit much superior false-alarm resistant and speed of response performance characteristics for the mass market. Second, NDIR CO.sub.2 sensors consume quite a bit of power by nature of its operation due to the need for a high temperature infrared source. Again, the extra power requirement for an added CO.sub.2 parameter in the dual CO.sub.2 /smoke fire detector hinders its application in the commercial fire detector market which requires systems with many detectors wired together and with mandatory and adequate standby battery power. The fact that the CO.sub.2 /smoke dual fire detector consumes quite a bit more power than the stand-alone smoke detector and hence requires a very large standby battery power supply prevents its application even in the commercial marketplace today. Thus the need still persists today for a low-cost, radioactive-free, reliable and false alarm resistant fire detector for the public at large.
The introduction of the CO.sub.2 parameter to assist the conventional smoke detector to eliminate false alarms and to provide faster response to the onset of fires as elucidated above represents a major step forward in the quest for the perfect fire detector. This progress was made based upon the observation that effluent gases, especially carbon dioxide gas, invariably accompany fire combustion in addition to the more familiar flame, heat and smoke. In addition to the work done on the CO.sub.2 parameter to improve the general performance of fire detectors, other approaches have also been actively considered. The most notable one was the use of multiple sensors and neural network methodology to develop the so-called "intelligent" fire detector. Such an intelligent detector was achieved using the analysis of signature patterns for discriminating fire detection with multiple sensors. Such an array of multiple sensors include carbon dioxide gas sensor, carbon monoxide gas sensor, oxygen gas sensor, temperature sensor, smoke sensor and a so-called "Taguchi" tin oxide sensor which detects the presence of organic volatile compounds including strange odors. While the development of an intelligent fire detector is prudent and necessary in the long run, the complexity and high cost for such an intelligent fire detector is not amenable to mass applications in the marketplace.
Based upon the discussion presented above, it is clear that a rapid, reliable, low-cost, radioactive-free and maintenance-free fire detector would be a most welcome addition to the imperfect world of fire detectors.